Oscillators – Electrical noise or random wave generator
Reexamination Certificate
1999-12-21
2002-03-26
Mis, David (Department: 2817)
Oscillators
Electrical noise or random wave generator
Reexamination Certificate
active
06362695
ABSTRACT:
BACKGROUND
1. Field
The invention relates to circuits which generate random bit sequences.
2. Background Information
Random bit sequences are sequences of binary signals lacking discernible patterns or repetition over time. Random bit sequences are useful in encryption operations and other electrical operations which employ unpredictable signal sequences. A “true” random sequence may be derived from thermal noise characteristics of electronic components.
FIG. 5
shows a prior art circuit
500
which employs thermal noise characteristics of electrical components to generate a random bit sequence. A resistor
502
and capacitor
504
pair provides a voltage signal to the positive input terminal of a differential amplifier
510
. Another resistor
506
and capacitor
508
pair provide a voltage signal to the negative terminal of the differential amplifier
510
. The differential voltage provided to the terminals of the differential amplifier
510
will vary randomly due to the thermal noise of the resistors. The voltage signal output by the differential amplifier
510
amplifies these random variations. For example, differential amplifier
510
may amplify the voltage between the positive and negative input terminals by a factor of 200 or more. The amplified differential voltage is input to a voltage controlled oscillator
512
(VCO). VCO
512
provides a periodic signal with a frequency that varies according to the voltage input from the differential amplifier
510
. The frequency of the VCO signal thus varies according to the random variations in the voltage signal provided to the VCO
512
. Circuit
514
converts the VCO signal into a ground-referenced signal which is supplied to the clock terminal of a D-style flip-flop
518
. Of course, a differential sampler circuit could be used in place of the conventional flip-flop, in which case the VCO signal could be applied to the sampler without converting to a ground-based signal with circuit
514
. Flip-flop
518
(or other sampler circuit) may be configured to propagate the signal at its D input terminal (the D signal) to its Q output terminal upon receiving an edge of VCO signal (this is often referred to as “latching” the D signal). The random variations in the frequency of VCO signal result in random variations in the timing of the edges of VCO signal. Therefore, flip-flop
518
latches the D signal at randomly-varying points in time. High frequency oscillator
516
(HFO) provides the D signal to the flip-flop
518
. D signal produced by HFO
516
transitions from logical low to logical high rapidly and periodically; flip-flop
518
will latch the D signal at logical low or at logical high randomly, according to the random variations in the timing of VCO signal edges. The latched D signal is output on terminal Q as random bit sequence
520
.
A disadvantage of circuit
500
is that amplifier
510
comprises numerous analog components. For example, the resistors and capacitors and the differential amplifier
510
may be implemented as analog circuits. Analog component designs tend to not scale well to higher frequency and lower voltage processes. Furthermore, the internal analog components of amplifier
510
may accumulate signals over time (such as so-called “1/f noise”) which result in deviations in the amplifier's desired performance. A further disadvantage of circuit
500
is that in order to maintain the randomness of the variation in the VCO output signal, differential amplifier
510
must be prevented from “railing”. Railing occurs when the input signals to the amplifier or the deviations resulting from “1/f noise” cause the amplifier output signal to reach a maximum predetermined low or high voltage. In other words, amplifier output signal may level off at a low or high DC voltage level. Once this happens, the VCO output signal frequency becomes more or less constant. The VCO output signal no longer reflects random variations in frequency. Consequently, the bit stream
512
provided at output terminal Q of flip-flop
518
will lose its random characteristics. Preventing the output of differential amplifier
520
from railing involves complex equalization circuitry. Such circuitry results in increased circuit cost, circuit size, and design time.
SUMMARY
A circuit includes a first oscillator having transistors to produce a first signal with random variations resulting from device channel resistance of the transistors.
REFERENCES:
patent: 4580109 (1986-04-01), Lockwood
patent: 5961577 (1999-10-01), Soenen et al.
Beiley Mark A.
Breisch James E.
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